Metal corrosion is inevitable, and has the potential to cause catastrophic pipeline and assets failure, damaging the environment and communities, injuring field operators, and causing costly downtime and waste in productivity.
To prevent and control corrosion by determining the specific problem and effective solution, at least five factors need to be considered:
- Environmental conditions such as soil resistivity, humidity and exposure to saltwater on various types of materials
- The type of product to be processed, handled or transported
- The required lifetime of the structure or component
- Proximity to corrosion-causing phenomena, such as varied metals, water or chemicals
- Appropriate mitigation methods
It is rare that a corroding structure or component will suffer from only one form of corrosion. The combination of metals used in a system, and the wide range of environments those systems are in, mean that different types of corrosion can attack. Even a single metal alloy can suffer corrosion in more than one form depending on its exposure to different environments at different points within a system.
How Corrosion Occurs
Nearly all forms of corrosion occur through the action of an electrochemical cell. The elements that are common to all corrosion cells are:
- An anode, where oxidation and metal loss occur
- A cathode, where reduction and protective effects occur
- Metallic and electrolytic paths between the anode and cathode, through which electronic and ionic current flows, and
- A potential difference that drives the cell – such as the characteristics of dissimilar metals, surface conditions, and the environment, including chemical concentrations
There are specific mechanisms that cause each type of corrosion attack, different ways of measuring and predicting them, and various methods that can be used to control them. The most common forms of corrosion are readily identifiable by ordinary visual examination and include:
Uniform corrosion – This is characterized by corrosive attack that occurs evenly over an entire, or large fraction of, a surface area. General metal thinning takes place until the system fails. Due to the waste it causes, this is the most important form of corrosion, but also is relatively easily measured and predicted, making disastrous failures relatively rare.
Pitting corrosion – A localized form of corrosion, cavities or holes are produced in the material. Pitting is considered to be more dangerous than uniform corrosion damage because it is more difficult to detect, predict and design against. A small, narrow pit with minimal overall metal loss can lead to the failure of an entire engineering system.
Crevice corrosion – A localized form of corrosion, this usually occurs when a stagnant solution rests in crevices, such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded coating, threads, lap joints or clamps.
Filiform corrosion – This is a form of crevice corrosion in which aggressive chemistry buildup occurs under a protective film that has been breached, such as painted or plated surfaces where moisture has permeated the coating. Lacquers and "quick-dry" paints are most susceptible to this.
Pack rust – If you see a bridge with rust packed between its steel plates, you are seeing pack rust, a form a localized corrosion typical of steel components that develop a crevice in an open atmospheric environment.
Galvanic corrosion – This refers to corrosion damage induced when two dissimilar materials are coupled and brought into electrical contact under water. When a galvanic couple forms, one of the metals becomes the anode and corrodes faster than it would by itself, while the other becomes the cathode and corrodes slower than it would alone.
Other types of corrosion that can require additional examination for verification include erosion, cavitation, fretting, intergranular, exfoliation, and dealloying, or selective leaching. Some types require verification by optical or electron microscopy, such as environmental cracking, stress corrosion cracking, corrosion fatigue, or hydrogen embrittlement.
Measuring Corrosion In Pipelines
The good news is that common forms of corrosion within a pipeline are easy to measure. A weighed sample – called a coupon – of the metal or alloy being measured is introduced into the pipeline and later removed after a set time interval. The coupon then is cleaned of all corrosion residue and reweighed. The weight loss is converted to a corrosion rate (CR) or metal loss (ML) rate. This process can help identify where, what kind, at what rate and why corrosion is occurring. Fortunately, this method simply requires an appropriately shaped coupon, a coupon holder, and a reliable means of removing the corrosion residue without disrupting the metal substrate. Coupons are an effective and inexpensive monitoring method for gauging the internal corrosion rate of pipelines, processing equipment and storage vessels.
Sentry® corrosion coupon holders are insertable and retractable to allow low- and high-pressure pipeline systems to be monitored while under normal operating conditions. The coupon can be removed safely without venting or draining of the process product, so there is no system downtime. These tools help ensure best practices for corrosion monitoring can be easily implemented to ensure systems can continue to operate at optimal capacity. Sentry monitoring products help you determine whether you need to maintain or replace equipment, prevent catastrophic leaks and system failures, extend equipment and asset life, and save your company money.
Learn more about Corrosion Monitoring products here.
This article was originally published on our website.
